This invention relates to a semiconductor laser drive device.
The semiconductor laser has excellent features that it is small in size, has high light emission efficiency and is capable of direct modulation. On the demerit side, however, it is sensitive to variations of the drive voltage and is fragile with respect to the application of an inverse or excessive voltage (drive voltage).
For these reasons, the prior art semiconductor laser drive device is provided with various protective functions. Such a prior art device is disclosed in, for instance, Japanese Patent Laid-Open No. 56-144,590. This prior art device, however, is provided mainly with means for the protection from a spike voltage upon application of a pulse voltage for modulating a laser beam from the semiconductor laser, or for stabilization of the laser output with respect to variations of the drive voltage. In other words, no substantial importance is attached to the protection with respect to the application of an inverse or excessive voltage.
When the semiconductor laser is used in a complicated system, it is connected to a power supply section or a control section via a cable or a connector. Thus, it can be readily installed and removed in case of assembling, testing and adjusting. When the semiconductor laser is removed from the power source section or control section, an inverse or excessive voltage is liable to be applied to it. Such inverse or excessive voltage has to be removed.
For this reason, the operation of the semiconductor laser drive device is controlled using a voltage discrimination circuit as shown in FIGS. 1A and 1B. In this arrangement, one of supply voltages to the semiconductor laser drive device is used as voltage to be discriminated 103 and compared to a reference voltage in comparator 101. The reference voltage is obtained as a division of another one 102 of supply voltages through a voltage divider consisting of resistors 104 and 105. Output 106 of comparator 101 is used as control signal for controlling the semiconductor laser drive device. In the case shown in FIG. 1A, the ground potential of voltage 103 is the same as the ground potential of the reference voltage. In the case shown in FIG. 1B, there is common mode noise 132 between the ground potential of voltage 103 and ground potential of the reference voltage. Reference numeral 131 in FIG. 1B designates equivalent voltage source of voltage 103.
With the above circuit, when voltage 103 has the opposite polarity or has a level in excess of a predetermined level, the operation of the semiconductor laser drive device is inhibited according to output 106 of comparator 101 to prevent the drive voltage from being applied to the semiconductor laser.
Such a voltage discrimination circuit, however, is a closed loop circuit. Therefore, when there is common mode noise 132 between voltage 103 and comparator 101 as shown in FIG. 1B, an excess common mode input is fed to comparator 101 to cause rupture thereof or detection sensitivity reduction thereof.